US 4596037 A
Apparatus for determining the location of a physical feature, such as a hole in a panel, relative to two coordinate axes by optically scanning across the panel and hole from a reference while measuring the distance of the hole from the reference and the length of the hole chord for each of a plurality of scans, then adding half the chord length to the respective distance to the hole edge and averaging the results for the plurality of scans. Scan lines used for the hole edge and chord measurement can be selected from among a plurality of scan lines having a predetermined chord length. The measuring apparatus can also be readily adjusted to accommodate a range of feature dimensions.
1. Apparatus for determining the location of a hole in a circuit panel comprising:
imaging means for generating a plurality of scan signals having magnitudes representative of features of the hole in the circuit panel;
means for generating pulses at a fixed frequency;
first accumulator means for accumulating said pulses during a first portion of each of the scan signals representative of a distance from a reference position to a boundary point of the hole;
second accumulator means for accumulating said pulses during a second portion of each of the scan signals representative of a distance across the hole;
means responsive to the magnitudes of the scan signals for gating said pulses to the appropriate one of the first or second accumulator means;
means for comparing each value accumulated by the second accumulator means to a preset minimum value and for discarding values accumulated by the first and the second accumulator means if the value accumulated by the second accumulator means is less than the preset minimum value; and
processor means for first selecting certain values accumulated by the first and the second accumulator means based on the expected size of the hole, to provide a representative sample of the features in the approximate center of the hole represented by the magnitudes of the scan signals, and for then processing only said selected values to determine the location of the hole in the circuit panel.
2. Apparatus as recited in claim 1 wherein said imaging means comprises:
a first video camera means associated with a pulse generating means, first and second accumulator means, gating means, comparing means, and processor means, as recited in claim 14, for scanning along a first set of parallel scan lines and for generating a first set of scan signals having magnitudes representative of features of the hole in the circuit panel; and
a second video camera means also associated with a pulse generating means, first and second accumulator means, gating means, comparing means, and processor means, as recited in claim 14, for simultaneously scanning along a second set of parallel scan lines which are substantially orthogonal to the first set of parallel scan lines scanned by the first video camera means, and for generating a second set of signals which also have magnitudes representative of features of the hole in the circuit panel.
3. Apparatus as recited in claim 1 wherein said hole is substantially circular, and wherein said processor means comprises:
means for adding the value in the first accumulator means to half the value in the second accumulator means for each of the selected values accumulated by the first and the second accumulator means, and for averaging said resulting values.
4. Apparatus as recited in claim 1 further comprising:
support means for movably supporting said circuit panel in relation to the imaging means.
5. Apparatus as recited in claim 4 wherein said support means comprises:
means for orientating said circuit panel relative to a home position of said support means.
6. Apparatus as recited in claim 5 further comprising:
means for determining the location of the imaging means relative to the home position of the support means.
Copyright International Business Machines Corporation 1984. The specification of this patent document contains material to which a claim of copyright is made. The copyright owner, assignee herein, has no objection to the duplication of the specification but reserves all other copyright rights whatsoever.
This invention relates generally to apparatus for optically measuring dimensions with video equipment and, more particularly, to such apparatus as used for automatically and accurately measuring dimensions of small features along two orthogonal axes.
During the manufacture of a printed circuit panel, a large number of through holes or vias is drilled by a multiple spindle drilling machine that is relatively moved from one drilling location to another over the panel. Dimensional accuracy of the hole placement must be assured and the panel must be carefully inspected to ensure that the drilling machine has been accurately positioned. Manual inspection of even a few selected holes makes the cost prohibitive. Accordingly, an automated system of measurement is required.
The automated optical measurement of relatively small dimensions is well-known. Most systems use a moving beam of light that traverses the surface of an object and changes reflectivity as contrasting features are encountered. An example of this method is United Kingdom Patent Specification No. 1,404,311 in which pulses are gated to an accumulator during the time light is reflected from a contrasting feature in the path traced by a small beam. The accumulated pulses become a measure of the feature dimension when the beam velocity is known or when scale marks are sensed during movement. In the case of holes or openings, one solution has been to gate pulses from a fixed frequency oscillator to an integrator or accumulator during the time a relatively moving light beam passes through the opening or impinges on a grid work of opaque lines during the movement. Again, the velocity of the beam is uniform and known to enable conversion to a dimension. Examples of this latter technique are taught in U.S. Pat. Nos. 2,447,024 and 3,546,671.
The holes drilled in printed circuit panels are becoming smaller in diameter as drilling capabilities are improved to enable more circuits and their vias to be placed within a unit area. In addition, the panels are being laminated with more layers so that the aspect ratio, length to diameter, of the hole is increasing. These factors make the usual approaches of light transmittance through the hole or direct reflectance for gating a counter unacceptable because the sensors are inaccurate or incapable at the smaller dimensions. A system using a vidicon camera and magnifying lens, such as that described in U.S. Pat. No. 3,551,052, is necessary to scan the drilled opening. This latter reference, however, is inadequate in determining the location of an opening with respect to a reference and cannot provide a dimensional output as a pulse number since it responds with only a single pulse during each traversal of a contrasting area.
In order to obtain accurate measurement of location, several scans are required to assure that irregularities at the edges of the opening are minimized in influence and that measurements can be taken with a high degree of accuracy, such as the distance of a reference to the edge of the opening or the distance across the opening on a particular scan line. The techniques heretofore known have been unable to perform these required measurements, thereby preventing the desired automating of hole inspections.
It is a primary object of this invention to provide an optical digital scanning system by which the distance of features of an object from a reference can be determined.
Another important object of this invention is to provide an optical scanning system that enables the location of an object feature to be determined relative to a reference by averaging the distance of a plurality of measurements obtained during a plurality of linear scans of the object and feature.
Yet another important object of this invention is to provide a video scanning system in which features on an object can be inspected as to location by selected linear scans of the object and computation of distances and averages thereof from a reference.
The foregoing objects are attained in accordance with the invention by providing video scanning means effective to scan an object a plurality of times in successive scans at a known velocity and sense the boundaries of contrasting features with the scan lines effective to gate pulses of fixed frequency into accumulator means in proportion to the time the scanning beam is sensing a particular feature and thereby convert the length of the scanned feature to a digital value. The scan lines are selected as to those effective for gating the pulses by either feature dimension or desired frequency of scan. The accumulated digital values can be combined after processing by computation means to provide distance values from a reference and deviations from an average.
The measuring system uses high-speed video sensing and includes optics to change magnification to vary the image size examined by the video scanning means and thus advantageously accommodate object features of varying sizes such as those encountered in drilled holes. Chord lengths of the holes are readily selected by setting minimum accumulator values for gated pulses. The use of multiple accumulators allows separate measurements and thus more versatility.
The foregoing and other objects, features and advantages of the invention will be apparent from the following, more particular, description of a preferred embodiment of the invention as illustrated in the accompanying drawing.
FIG. 1 is a perspective view of apparatus constructed in accordance with the principles of the invention for measuring the locations of holes in a circuit panel;
FIGS. 2a-2c are a diagram of an image of a circuit panel and hole as seen with the apparatus in FIG. 1 and waveforms produced from said image;
FIG. 3 is a diagram of a circuit for determining distances with data from the apparatus shown in FIG. 1;
FIG. 4 is a diagram of an image of a circuit panel hole illustrating measurement limits thereof; and
FIG. 5 is a summary flow chart for processing measurement data obtained with the apparatus shown in FIG. 1.
FIGS. 6-12 are flow charts showing a data processing technique in detail for obtaining measurement results.
Referring to FIG. 1, the measuring apparatus comprises generally a table assembly 10 movable along the Y axis carrying a circuit panel 11 having therein drilled holes 12 that are viewed by a pair of overhead video cameras 13, 14 supported for movement along an X axis on a movable carriage assembly 15. Table 16 is mounted for reciprocating motion on guide rods 17 supported in blocks 18. A motor 19 and lead screw 20 can be selectively energized to drive the table to a desired position along the Y axis. Magnitude and direction of motion along that axis is indicated by signals in well-known phase quadrature from a photosensitive optical transducer 21 sensing opaque marks on linear scale 22 secured to table 16. Circuit panel 11 is held in position on the table by locating vee blocks 23.
Drilled holes 12 and panel 11 are viewed by a pair of identical video cameras 13, 14 each oriented along one of the orthogonal X and Y axes to provide images of a selected hole that are displaced 90° from each other. Commercially available video cameras may be used such as Model LSV-1.5/DBA-1 from Sierra Scientific Corp. in Mountain View, Calif. The cameras are supported on a carriage 30 that is movable along the X axis on guide rod 31, supported in hanger blocks 32, only one of which is shown. Camera carriage 30 is moved on guide rod 31 by a selectively operable motor and lead screw, not shown, similar to that for table 16. A linear scale 33 is sensed by photosensitive optical transducer 34 to provide output signals in phase quadrature that can be processed in a well-known manner to indicate magnitude and direction of motion. A selected hole 35 to be viewed is illuminated by light from a source, not shown, delivered through bundles 36 of optical fibers. The selected hole is viewed by cameras 13 and 14 through a collimating lens 37, a beam splitter 38 and mirror 39 and through respective magnifying lenses 40 and 41. Lens 37 is supported in a cylindrical housing or quill 42 that is moved vertically from a retracted position into contact with panel 11 to assure proper focus when video camera images are to be generated.
The inspection system shown is under the control of a larger computer system for achieving automatic inspection procedures. The larger system may include one or more central processing units for controlling positioning of the table, cameras shutters and lens focusing among other operations. The individual hole calculations, selection of useful data, and averaging of data is under the control of a microprocessor 60, to be subsequently described.
Inspection of selected drilled holes is undertaken to determine the coordinate locations of their centers with respect to a home position. This is done initially by placing panel 11 on table 16 and moving the table to a "home" position that is a mark on one of locator blocks 23. All other locator blocks can also be checked. The video measuring apparatus is adjusted to provide a 0--0 coordinate location for the home position at the monitor screen center by counting pixels to its midpoint. Thereafter, the motion of the table and camera carriage assemblies 10 and 15 can be measured by the photosensitive transducers 21, 34 and respective scales 22, 33.
A hole whose diameter and location are to be determined is imaged by both orthogonally oriented video cameras oriented so that the measurements can be orthogonally defined. The measurements can be done simultaneously with the described arrangement to achieve efficiency. Since both camera systems operate in a similar manner, only the system for the X axis will be described in detail.
An image of hole 35 (FIG. 1) as seen by vidicon camera 13 and seen in a monitor, not shown, is shown in FIG. 2a. The camera is typically a non-interlacing type that has 525 scan lines of video in a frame. The scan lines 45 each start from the left and move right across the field of view. The size of the imaged drilled hole is determined by the magnifying lens 40 for the X axis and this enables a range of hole diameters to be scanned merely by changing the magnification. In the measuring system of the invention, the location of the center of hole 35 is defined with respect to the left edge 46 of the frame as seen in FIG. 2a. This requires that the distance from the left frame edge to the center of the hole has to be measured. The distance that the hole center lies from the frame edge 46 is measured by gating fixed frequency pulses, such as from an oscillator to two different accumulators or counters during the generation of a scan line. A vidicon operating at 15 Hz completes a scan line each 126 usec. Since the scan line velocity is a known value and the pulse frequency is fixed, the accumulator values represent distance from the time each accumulator is gated until blocked.
A diagram of a circuit and microprocessor for determining the distance from the reference edge to a hole edge and the center of the hole is shown in FIG. 3. The microprocessor shown is a mode1 6800 available from the Motorola Corporation at Schaumburg, Illinois. The circuit uses multiple inspection scans that produce signal levels shown in FIGS. 2b and 2c as gating signals to control the accumulation of pulses representing distance. In FIG. 2b, the waveform shown represents that of a single horizontal scan line 47 across the approximate midpoint of the monitor view of a portion of the circuit panel and drilled hole 35. The negative excursion 48 represents the horizontal synchronizing pulse for the scan line; signal level 49 represents a dark surface; and level 50 represents a reflective or light surface. Thus, the scan line trace, after synchronization, rises to level 50 indicating a light or reflective panel surface, then later falls to a dark level 49 as the hole edge is encountered where it remains until rising again after leaving the hole and again sensing panel surface. These levels are used to gate oscillator pulses into different counters. A further gating signal is shown in FIG. 2c where a counter enable signal reaches an activating level at the termination of horizontal synchronizing pulse 47 in FIG. 2b.
In FIG. 3, an oscillator 52 of fixed frequency, such as 10 Mhz, has its output connected as one input to AND gate 53 and one input to AND gate 54. When AND 53 is conditioned by a counter enable signal from FIG. 2c, indicating that a scan line has started, pulses will be transmitted to counter 55. Counter 55 will accumulate pulses until the transition in the light level to the dark level or from level 50 to level 49 on the trace in FIG. 2b. At that transition at terminal 56, a set signal to latch 57 will lock the counter value in the latch. This value represents the distance from the left side 46 of the image in FIG. 2a to the left edge of hole 35. The second input to AND 54 is from an inverted vidicon output and is effective from the stable horizontal synchronization pulse 48 at the dark levels 49 to condition AND gate 54 so that oscillator pulses are directed to counter 58. During the time that the dark level is present at AND 54, counter 58 will accumulate pulses and, as the scan line senses the far edge of the hole, the dark level signal will terminate signaling computer 60 so that counter 58 contains a value representative of a chord length of hole 35. The dark-to-light signal causes the microprocessor 60 to initiate a comparison of the value in counter 58 with a preset value.
During the generation of scan lines, it is desirable that only selected scan lines be considered for measurement. At the conclusion of a scan line, the accumulated count in counter 58 is compared at compare circuit 59 with a preset value from microprocessor 60. If the accumulated value of counter 58 is less than the preset value, a reset signal is transmitted from compare circuit 59 to both counter 58 and latch 57. This action erases the accumulated values in counter 58 and latch 57 so that the scan line data is not used. A value that is too small in counter 58 indicates either that the scan line did not encounter a hole 35 in FIG. 2a or that the chord portion across the hole was too short to be of interest. Thus, the preset value is a technique of screening acceptable scan lines through the setting of minimum lengths.
If the accumulated count in counter 58 is equal to or greater than the preset value in comparator 59, a signal is sent to microprocessor 60 to store the value of latch 57 and the value of counter 58 for that particular scan line within the microprocessor for later computation, such as adding the value of the latch 57 to half the value of the counter 58 to determine, for that scan line, the distance of the X-coordinate of the center of the hole 35 from the frame edge 46 of the video frame. Counters 55 and 58 and latch 57 are then reset for the next scan line. The 525 scan lines provided by the vidicon are in excess of the number required to make a reliable measurement. Therefore, the microprocessor is programmed to use one out of any desired number such as 5 or 10 scan lines, and is programmed to add the value of the latch 57 to half the value of the counter 58 only for each of the selected ones of said scan lines 45 resulting in a number of values which may be averaged by the microprocessor to significantly improve the determination of the distance of the X-coordinate of the center of the hole 35 from the frame edge 46 of the video frame. The vertical and horizontal sync pulses are used for counting frames and scan lines. This affords a technique of reasonably limiting the number of scan lines to be reviewed. As mentioned previously, the preset value placed by the microprocessor 60 in compare circuit 59 can be nearly any desired value. One technique of determining this value is shown in FIG. 4 and is to determine the theoretical lengths of chords between the intersections of a pair of diameters with the circumference when oriented at 45° with a third diameter parallel to the scan lines. The resulting value can be used as a minimum chord length and changed, of course, for different sizes of holes.
The invention offers significant improvement in determining the location of the center of a hole by using a plurality of linear scans to sense the hole edge. Selection and use of a predetermined number of scans, such as sixteen, permits the averaging of results that provide greater accuracy. A deviation of the computed hole center location from the average for each of the plurality of chords is easily calculated, and those values widely varying or beyond established limits can be discarded. Irregular hole edges can be detected in this manner to prevent erroneous conclusions.
A computer data flow diagram is shown in FIG. 5 that describes the video portion of the inspection process. Sixteen chords are illustrated as the total used for a measurement. The number, however, can be changed to other values, as desired.
As mentioned above, the automatic hole inspection process is controlled by a general purpose computing system. The microprocessor is a part of and communicates with this system for the computation of specific hole locations data. A preferred method of implementing the inspection system is shown in FIGS. 6-12; other techniques of the implementation can readily be devised by others skilled in the art. Source code by which the inspection system is preferably enabled is shown in appendix A.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. ##SPC1## ##SPC2## ##SPC3##